The present invention relates to a method of controlling strip crown in planetary rolling of metallic material on a planetary mill for improving strip crown of planetary-rolled strips by preventing the occurrence of edge-drop and high spot of the strip crown.
In hot rolling of a strip, there has been used a rolling mill such as a tandem mill, a steckel mill, a planetary mill or the like. The hot strip manufactured by such a rolling mill always has a thickness deviation along the width of the strip which is referred to as a strip crown. The strip crown is classified into a center crown formed by slightly increasing the thickness in the central region of the strip and an edge drop formed by sharply decreasing the thickness in the edge region of the strip. In order to improve the strip crown, in the tandem mill, there have been known some methods such as a method using roll benders for work rolls and backup rolls in finishing the roll stands, a method using a six roll mill referred to as a HC mill and a method using tapered rolls. It has been understood from studies about the tandem rolling that the final strip crown is affected by only the last stand of the finishing roll stands rather than by the forward stands. In view of such an understanding, the last two or three stands of the finishing roll stands in the tandem mill have been modified to provide a shape control function and resulted in substantial effects.
The planetary mill comprises a pair of backup rolls having a large diameter and a plurality of planetary rolls having a small diameter arranged around each of the backup rolls. A slab is rolled by one to three sets of feeding rolls and then rolled by the planetary rolls. The slab is subjected to a small reduction several times by the planetary rolls, resulting thereby in a large total reduction of more than 90% by the planetary mill. Therefore, the planetary mill performs the function of the latter half of roughing roll stands and the finishing roll stands of the tandem mill.
The planetary mill is subjected to a very small roll separating force in spite of a large total reduction and the center crown of the planetary-rolled strip is very small owing to the backup rolls of the large diameter and the total crown depends on the edge drop.
The conventional feeding rolls used on the planetary mill line have straight crowns (FIG. 6) or curved crowns defined by a quadratic curve for correction of elastic deformation of the rolls or convex crown defined by a sine curve (FIG. 7).
The characteristic of the crown of the planetary mill is described, for example, in "Journal of the Japanese Society of the Technology of Plasticity" Vol 22, No, 247, 1981-8, P 839-846 "the shape and characteristic of planetary-rolled strip". There is described that the strip crown is affected by many factors. Furthermore, Japanese Patent Application laid open Publication No. 51-66263 discloses a planetary mill of crown controllable type comprising double groups of planetary rolls arranged around the backup rolls, the roll crown of which is adjustable.
The aforementioned "Journal of the Japanese Society of Technology Plasticity" does not disclose any teaching for improving the strip crown. The arrangement of the rolling mill described in Japanese Patent Application laid open Publication No. 51-66263 can not be adapted for the Sendzimir type planetary mill having single group of planetary rolls. Moreover, when the arrangement of the rolling mill is adapted to the existing planetary mill having double groups of planetary rolls, it is necessary to modify on a large scale and such modification is practically difficult.
When rolling the slab by the planetary mill, the thickness of the rolled strip having for example five feet width, sometimes locally increases in a region width 50-150 mm from the edge of the strip. It is called a high spot. This phenomenon occurs when a slab having a flat crown, after leaving the feeding rolls, is fed to the planetary rolls. The cause of such a phenomenon has not been made entirely clear. It is conjuctured that the phenomenon is caused by bending of comparatively slender planetary rolls, but the cause is still unclear. In planetary rolling, when the thickness of the center of the rolled strip is increased, the rolled strip shows center buckling. And when the thickness of the rolled strip is locally increased, the rolled strip tends to show guarter buckling.
Such a phenomenon is entirely contrary to the usual rolling wherein a thinner portion is elongated and shows buckling.
When the planetary-rolled strip having the local thickness deviation along the width thereof is rerolled, a complex localized elongation of the strip occurs and such a localized elongation could not be sufficiently corrected by the profile control technique so that the locally elongated strip is not used as a rerolling material.
As aforementioned, the conventional method of rolling on the planetary mill line has a problem such that the edge drop of the strip crown could not be decreased and the local increase of the thickness, i.e. high spot, could not be prevented.